The present invention relates to protective relay circuits for use in a-c power systems and more particularly to ultra high speed circuits for transmission line protection.
Distance relays are well known in the art of protective relaying. For example, reference may be had to to "The Art and Science of Protective Relaying", Mason, published by John Wiley & Son, Inc. (1956), particularly to Chapter 14. Prior art distance relays include those which employ a phase comparator method of measurement. Exemplary circuits for phase comparator distance relays are disclosed in U.S. Pat. No. 4,034,269, entitled "Protective Relay Circuits", issued July 5, 1977 to S. B. Wilkinson. This patent is assigned to the assignee of the present invention and is hereby incorporated by reference into the present application.
In the phase comparator method of measurement, the relay operates by measuring the angle between two or more phasors derived within the relay. Briefly, this is accomplished by the use of an a-c coincidence circuit in combination with a timer circuit. For example, in the case of the phase distance relay of FIGS. 6 and 7 of my previously referenced U.S. Pat. No. 4,034,269, a coincidence detector 286 develops an output signal when phase coincidence exists between a polarizing signal (V.sub.AB) and an operating signal [(I.sub.A -I.sub.B)Z.sub.R -V.sub.AB ]. A characteristic timer 288 then measures the duration of the phase coincidence and operates when phase coincidence occurs for a predetermined minimum time equal to the relay operate time. It is to be appreciated that, for faults within the intended reach of the relay, the phase coincidence is greater than 1/4 cycle or 90.degree.. Thus, for 60 cycle protective relaying applications, where 1/4 cycle equals 4.167 ms, the timer is ordinarily set at 4.167 ms.
A traditional problem with distance relays is the compromise between operating time and the need to prevent operation on heavy load flow or load swings. For example, on long lines, 120.degree. characteristic timer settings are often used to preclude operation on load swings that would enter the standard circular characteristic when using a 90.degree. timer setting. This 120.degree. characteristic timer setting establishes a minimum operating time of 5.5 ms for all fault incidence angles.
Another problem with conventional prior art distance relays involves the undesired tripping for faults which occur very close to the relay on the bus behind it. Because of the load flow over the line in a situation such as this, the operate signal V.sub.OP leads the initial polarizing voltage V.sub.POL by substantially more than 180.degree., causing the angle between V.sub.OP and the steady state V.sub.POL to be much less than 90.degree.. This condition could cause the operation of a conventional, prior art distance relay. A further problem with conventional prior art distance relays concerns their inability to immediately clear high level faults due to their slower operating speeds.
In some prior art distance relay designs, a forward offset is employed to minimize the probability of relay operation under load conditions. This forward offset could be employed both to minimize the angular separation of the operate and polarizing signals, and to preclude operation of the distance relay when the impedance "seen" by the relay due to an out of step condition, passes through the line impedance. For this application, the offset impedance should be precisely equal to the line impedance. If it is less, the relay will operate for power swings passing through the line impedance near the end of the line; and if it is larger, there is a danger that the polarizing voltage will reverse during the fault on the remote bus and cause a false operation when the fault is cleared and the operate voltage returns to its phase position, which would be approximately in phase with two reverse polarizing voltage.
It is therefore, one object of the present invention to increase the overall power system reliability by providing a protective relay circuit which causes the fast clearing of severe faults.
Another object of the present invention is the provision of the protective relaying circuit which enhances the security of the transmission line protective system by preventing tripping upon occurrence of faults which occur close to, but behind the relay.
Still another object of the present invention is the provision of a protective relay circuit which eliminates the possibility of undesired operation on clearing external faults, or power swings passing through the line impedance, and particularly for those difficult ones near the end of the line.